levenshtein_distance in ruby - ruby

Question

I wrote levenshtein_distance as below to calculate the distance between two strings:
def min3(a, b, c)
if a < b && a < c then
a
elsif b < c then
b
else
c
end
end
def levenshtein_distance(s, t)
m = s.length
n = t.length
return m if n.zero?
return n if m.zero?
d = (0..m+1).to_a
x = nil
s.each_char.each_with_index do |ch1, i|
e = i + 1
t.each_char.each_with_index do |ch2, j|
cost = ch1 == ch2 ? 0 : 1
x = min3(d[j + 1] + 1, e + 1, d[j] + cost)
d[j] = e
e = x
end
d[m] = x
end
x
end
When the two strings are different, it gives an error message:
NoMethodError - undefined method `+' for nil:NilClass
The error detects line:
x = min3(d[j + 1] + 1, e + 1, d[j] + cost)
I thought this was due to index surpassing the defined d's limit. But enlarging the length of d doesn't solve this problem.
Is there something I missed in implementing the algorithm?
this is the case that I tested on irb
irb(main):052:0> levenshtein_distance("a", "abc")
NoMethodError: undefined method `+' for nil:NilClass
from (irb):24:in `block (2 levels) in levenshtein_distance'
from (irb):22:in `each_char'
from (irb):22:in `each_with_index'
from (irb):22:in `block in levenshtein_distance'
from (irb):20:in `each_char'
from (irb):20:in `each_with_index'
from (irb):20:in `levenshtein_distance'
from (irb):52
from /usr/bin/irb:12:in `<main>'

Answer 1

I've rewritten the algorithm in accordance to Wikipedia:
def ld(s, t)
v0 = (0..t.length).to_a
v1 = []
#p v0
s.chars.each_with_index do |s_ch, i|
v1[0] = i + 1
t.chars.each_with_index do |t_ch, j|
cost = s_ch == t_ch ? 0 : 1
v1[j + 1] = [v1[j] + 1, v0[j + 1] + 1, v0[j] + cost].min
end
v0 = v1.dup
#p v1
end
v0[t.length]
end
It seem to work. Also you can as well uncomment p v1 and p v0 to see how the vectors change on each iteration.